510 Not Extended

RFC2774 is an experimental RFC, that introduces a mechanism to allow
developers to extend the HTTP protocol with vendor-specific features in a
safe namespaced way.

Around this time many new protocols and systems started to get built on top
of HTTP, and the authors might have felt there was a need to do this in a
namespaced way.

This is not unlike XML namespaces, where it’s possible for one standards
organization to define a new format (for example ATOM), and other corporations
wanting to add new properties, avoiding the possibility of collision if a
second vendor also introduced a property with the same name.

As far as I know this extension to HTTP can be considered dead.

The 510 Not Extended statuscode could be used when a server requires that
a client uses an ‘extended HTTP request’, but didn’t. When a server emits
this response, it should also tell the client how to extend the request,
but the specification doesn’t specify a format for this.

Because this error is a ‘client error’, it should probably have had a 4xx code,
not a 5xx one.

I would not recommend for anyone to use this.

Should I use this?

No. RFC2774 is a very obscure extension to HTTP, and as far as I know has never
really been picked up. If you feel you have a need for this, I believe there is
likely to be a different way to extend HTTP. Perhaps via content negotiation, or
just different kinds of messaging from client to server.